Process for fabricating a monolayer or multilayer metal structure in liga technology, and structure obtained

ABSTRACT

The invention relates to a process for fabricating a monolayer or multilayer metal structure in LIGA technology, in which a photoresist layer is deposited on a flat metal substrate, a photoresist mold is created by irradiation or electron or ion bombardment, a metal or alloy is electroplated in this mold, the electroformed metal structure is detached from the substrate and the photoresist is separated from this metal structure, wherein the metal substrate is used as an agent involved in the forming of at least one surface of the metal structure other than that formed by the plane surface of the substrate.

This application is a divisional of U.S. application Ser. No. 11/717,773filed Mar. 14, 2007, whose contents are hereby incorporated by referenceherein in their entirety.

The present invention relates to a process for fabricating a monolayeror multilayer structure in LIGA technology, and to a novel monolayer ormultilayer metal structure that can be obtained by this process.

DGC Mitteilungen No. 104, 2005 mentions the use of the technology calledLIGA (Lithographie Galvanik Abformung [Lithography, Electroforming,Molding], a method devised by W. Ehrfeld of the Karlsruhe NuclearResearch Center, Germany) for the fabrication of high-precision metalparts for timepieces, such as for example anchors or escapement wheels.This process has the drawback of requiring expensive equipment, namely asynchrotron, to generate the X-ray irradiation. It therefore cannot bewidely used in the watchmaking industry.

A. B. Frazier et al., Journal of Microelectromechanical Systems, 2, 2,Jun. 1993, describes the fabrication of metal structures byelectrodeposition of metal in molds made of a polyimide-basedphotoresist, these being produced by means of a process using atechnology called UV-LIGA, similar to the LIGA technology but with UVillumination instead of irradiation with X-rays.

The process used for fabricating monolayer metal structures comprisesthe following steps:

-   -   creation on a silicon support wafer of a sacrificial metal layer        and a seed layer for the electrodeposition;    -   deposition of a photosensitive polyimide layer by spin coating;    -   UV illumination through a mask corresponding to the desired        impression;    -   development, by dissolving the nonirradiated parts so as to        obtain a polyimide mold;    -   electrodeposition of nickel or copper in the open part of the        mold up to the height thereof; and    -   removal of the sacrificial layer and separation from the support        wafer of the metal structure obtained by electroforming, and        removal of the polyimide mold.

That document describes monolayer metal structures such as copper ornickel gearwheels manufactured using this process (cf. A) and B) pp.89-91 and FIGS. 3-7).

Frazier et al. also discloses a process for fabricating bilayer metalstructures, comprising the following steps:

-   -   creation on a silicon support wafer of a sacrificial metal layer        and a seed layer for the electrodeposition;    -   deposition of a photosensitive polyimide layer by spin coating;    -   illumination with ultraviolet rays through a mask corresponding        to the desired impression;    -   development, by dissolving the nonirradiated parts so as to        obtain a polyimide mold;    -   electrodeposition of nickel in the open part of the mold up to        the height of the latter, so as to obtain a substantially plane        upper surface;    -   vacuum vapor deposition of a thin chromium layer;    -   deposition of a photosensitive polyimide layer on this thin        chromium layer by spin coating and removal of the thin chromium        layer using a hydrochloric acid solution;    -   UV illumination through a new mask corresponding to the desired        impression, development, by dissolving the nonirradiated parts        so as to obtain a new polyimide mold, and electrodeposition of        nickel in the open part of the mold; and    -   removal of the sacrificial layer, separation of the metal        structure obtained by electrodeposition from the support wafer        and removal of the polyimide mold.

That document describes the use of this process for fabricating a metalplate surmounted by a protuberance of parallelepipedal general shape (V.and FIG. 9, page 92), the second layer being entirely superposed on thelarger-area first layer.

The processes described by Frazier et al. do not allow machiningoperations to be carried out on the fabricated structures before theyhave been detached from the silicon support wafer. This is because thesupport wafer is too fragile to withstand the mechanical forcesgenerated by machining. If a machining operation is necessary, itrequires each fabricated metal structure to be precisely positioned andimmobilized. This makes the fabrication complicated and difficult toimplement.

EP 0 851 295 discloses another UV-LIGA process for fabricatingmultilayer metal structures, such as a toothed wheel surmounted by abilayer pinion (example 1) or a trilayer heat flux microsensor (example3). That process is similar to the one described by Frazier et al. andalso has the abovementioned drawbacks.

It has already been proposed in U.S. Pat. No. 5,766,441 to use a coppersubstrate from 500 μm to 2 mm in thickness, said document specifyingthat these thicknesses have no influence on the process described. Therehas also been described, in US 2005/0056074, a LIGA process forfabricating inserts for the compression molding of metals using astainless steel substrate. These documents do not propose the use ofthis substrate as a shaping element for at least one surface of themetal structure other than that formed by the plane surface of thesubstrate.

The object of the invention is to find a process for fabricating a metalstructure that does not have the abovementioned drawbacks and that opensthe way to as yet unexplored possibilities in the field of LIGA.

For this purpose, the subject of the present invention is a process forfabricating at least one monolayer or multilayer metal structure in LIGAtechnology, in which a photoresist layer is deposited on a flat metalsubstrate, at least one photoresist mold is created by irradiation orelectron or ion bombardment, a metal or alloy is electroplated in thismold, the electroformed metal structure is detached from the substrateand the photoresist is separated from this metal structure, wherein themetal substrate is used as an agent involved in the forming of at leastone surface of the metal structure other than that formed by the planesurface of the substrate.

The Applicant has in fact discovered that the use of a bulk metalsubstrate, instead of a silicon support wafer covered with a sacrificialmetal layer and with a seed layer, makes it possible to machine in situthe structure resulting from the electroforming step before it isdetached from the substrate. Thus, it is possible, when fabricating alarge number of structures (or parts) on one and the same substrate, forthese structures to be machined collectively before they are detachedfrom the substrate when they have been very accurately positionedthereon thanks to the mask used for forming the impressions on theresist layer. The additional step of positioning and immobilizing eachstructure after detachment from the substrate, for the purpose ofsubsequent machining, is therefore no longer necessary. Apart from thesimplification achieved, this process benefits most particularly fromthe very great precision in positioning parts on the substrate,something which could no longer be subsequently reproduced to such adegree.

In addition, in the case of the fabrication of multilayer metalstructures, the use of a bulk metal support allows leveling by machining(abrasion and polishing) at the end of each electrodeposition step so asto obtain a plane upper surface or the desired thickness. This has theeffect of improving the quality of the multilayer metal structureobtained since the subsequent electroplating is performed in a moreregular manner and allows better thickness control on a plane surfacethan on a surface having irregularities.

Thus, as will be observed throughout the description that follows, inthe process according to the present invention, the substrate plays therole of an agent involved in the forming of at least one surface of themetal structure other than that formed by the plane surface of thesubstrate.

The irradiation in such a LIGA process may be X-ray irradiation orUV-ray irradiation, by illumination in normal mode through a mask, byillumination in laser mode, in order to create an array of cured zones,or by laser ablation.

Preferably, the LIGA technology is a UV-LIGA technology, that is to saya LIGA technology using UV irradiation.

The invention relates to a process for fabricating a monolayer machinedmetal structure in UV-LIGA technology, which comprises the followingsteps:

-   -   a) a bulk metal substrate is coated with a photoresist layer;    -   b) if necessary, the photoresist layer is heated in order to        evaporate the solvent;    -   c) the photoresist layer is exposed to UV irradiation of 100 to        2000 mJ/cm² measured at a wavelength of 365 nm, through a mask        corresponding to the desired impression;    -   d) if necessary, in order to complete the photocuring or the        photodecomposition, the layer obtained after step c) is        annealed;    -   e) development takes place by dissolving the uncured or        photodecomposed parts;    -   f) a metal or an alloy is electrodeposited in the open parts of        the photoresist mold;    -   g) the electroformed metal structure is leveled by machining, so        as to obtain a plane upper surface;    -   h) if necessary, one or more other machining operations are        carried out on the upper face of the electroformed mechanical        structure; and    -   i) the metal structure and the cured photoresist are detached,        by delamination, from the bulk metal substrate and the cured        photoresist is separated from the machined monolayer metal        structure.

The bulk metal substrate is a bulk metal plate with a thickness ingeneral of 1 to 5 mm, of any shape, for example cylindrical orparallelepipedal, the extent of the upper surface of which is chosenaccording to the number of structures fabricated on any one substrate,in general from 1 to 5000, in particular from 10 to 3000. This plate isformed from a metal and/or a conducting alloy capable of seeding(starting) the electroforming reaction by acting as cathode. Forexample, it may consist of copper, brass or stainless steel. Preferably,it consists of stainless steel.

The upper surface of the bulk metal substrate, intended to be in contactwith the electrolyte bath, may be polished or textured, for example bymicropeening, chemical or mechanical etching or by laser. In the case ofetching the metal substrate, it is then possible to obtain at least onesurface element in relief resulting from the etching made on said metalsubstrate. This is absolutely novel, as etched features are alwaysformed by removal of material.

The etching of the substrate is carried out with a mask that ispositioned on the metal substrate using the same positioning system asthe mask used to form the apertures of the mold defining the shape ofthe metal structure produced by LIGA. Since the two masks have commonpositioning references, the etched features formed on the surface of themetal structure may be positioned very precisely with respect to theperimeter of the metal structure produced by electrodeposition.

The bulk metal substrate is degreased and prepared for theelectroforming by a suitable treatment. When this substrate consists ofstainless steel, a suitable treatment consists for example of analkaline degreasing step followed by an acid neutralization step, inorder to passivate its surface, rinsing with distilled water and drying.

The photoresist is either a negative photoresist, based on a resin thatcan be cured under the action of UV radiation in the presence of aptotoinitiator, or a positive photoresist, based on a resin that candecompose under the action of UV radiation in the presence of aptotoinitiator. The negative photoresist is for example based on anepoxy resin, an isocyanate resin or an acrylic resin. An advantageousepoxy resin is the octofunctional epoxy resin SU-8 (Shell Chemical). Itis used in general in the presence of a ptotoinitiator chosen fromtriarylsulfonium salts, for example those described in patents U.S. Pat.Nos. 4,058,401 and 4,882,245. The positive photoresist is for examplebased on a novalac-type phenol-formaldehyde resin in the presence of aDNQ (diazonaphthoquinone) ptotoinitiator.

The photoresist may be deposited by spin coating or by anothertechnique, such as for example dip coating, roll coating, extrusioncoating, spray coating, or lamination (for dry films, for example basedon an acrylic resin). The preferred coating technique is spin coating.

The maximum photoresist thickness for inducing the desired effect(photocuring or photodecomposition) under the irradiation conditions ofstep c) is typically 1 mm in the case of UV-LIGA. The thickness of thephotoresist layer that can be coated in one application is typically 150μm, depending on the spin coating technique. The bulk metal substratewill be coated with photoresist one or more times, depending on itsdesired thickness.

The conditions under which the photoresist is possibly heated, in orderto remove the solvent in step b), are chosen according to the nature andthe thickness of the photoresist as per the instructions from itsmanufacturer. For a photoresist based on an SU-8 epoxy resin of 140 μmthickness, step b) consists for example in heating at 65° C. for 5 to 10minutes and then at 95° C. for 30 to 60 minutes. For a photoresist basedon a dry acrylic film, this heating step to evaporate the solvent isunnecessary.

If the photoresist has to be applied several times and if thephotoresist has to be heated to evaporate the solvent, step b) will becarried out following step a) after the first application of thephotoresist, and steps a) and b) will be repeated the number of timesnecessary.

Step c) consists in exposing the photoresist layer to UV irradiation of100 to 2000 mJ/cm², measured at a wavelength of 365 nm, through a maskcorresponding to the desired impression. This irradiation inducesphotocuring of the resin (negative photoresist) or photodecomposition ofthe resin (positive photoresist).

Step d) consists, if necessary in order to complete the photocuring orphotodecomposition of step c), in annealing the layer obtained afterstep c).

Step e) consists in developing the structure, by dissolving thenonirradiated parts (negative photoresist) or the irradiated parts(positive photoresist) using a suitable aqueous solution or a solvent,chosen depending on the nature of the photoresist per the instructionsfrom its manufacturer. Examples of suitable aqueous solutions areweak-based alkaline solutions, for example sodium carbonate solutions,and examples of suitable solvents are GBL (gamma-butyrolactone), PGMEA(propylene glycol methylethyl acetate) and isopropanol. As developmentsolvent or solution, it is advantageous to use PGMEA for an epoxy resinand a 1% sodium carbonate solution or isopropanol for an acrylic resin.

Step f) consists in electrodepositing a metal or an alloy in the openparts of the photoresist mold, up to a defined height equal to or belowthe height of the photoresist mold using the bulk metal substrate ascathode.

Step g) consists in leveling, by machining, so as to have a plane uppersurface. This is made possible by the presence of the bulk metalsubstrate. This operation also makes it possible to ensure that there isperfect parallelism between the two surfaces of the electrodepositedmetal structure.

It is frequent to use, as metal for the electroforming, nickel, copper,gold or silver and, as alloy, a gold-copper, nickel-cobalt, nickel-ironor nickel-manganese alloy. The electroforming conditions, especially thecomposition of the baths, the geometry of the system, the voltages andthe current densities, are chosen for each metal or alloy to beelectrodeposited according to the techniques well known in theelectroforming art (cf. for example G. A. DiBari, “Electroforming”,Electroplating Engineering Handbook, 4th Edition, edited by L. J.Durney, published by Van Nostrand Reinhold Company Inc., New York, USA1984).

The machining leveling operation is generally carried out by abrasionand polishing, thereby producing a plane upper surface with surfaceirregularities not exceeding about 1 μm.

Step h) consists, if necessary, in order to obtain the desiredstructure, in carrying out other machining operations on the upper faceof the electroformed mechanical structure, such as for example beveling,etching or decorative machining.

Step i) consists in detaching the machined metal structure from the bulkmetal substrate by delamination and in separating the photoresist fromthe machined metal structure. Before separating the photoresist,etching, surface-treatment and mechanical or laser marking operationsmay where appropriate be carried out on the detached metal structure.

The lower face of the metal structure detached by delamination from theupper face of the bulk metal substrate reproduces the surface finish ofthis upper face. Thus, it will either be textured (if the upper face ofthe metal substrate is textured, for example by etching or micropeening)or have a polished appearance, polished to the desired degree ofpolishing (if the upper face of the metal substrate has been polished tothe desired degree of polish). In the latter case, to the naked eye, thepolished appearance of the surface of the lower face of the structurecannot be distinguished from the polished appearance obtained, whereappropriate, by polishing the surface of the upper face. When examinedunder an optical microscope with a magnification of 50 times, withsuitable illumination and a certain orientation, or under a scanningelectron microscope, or using topographic surface analysis systems, adistinction can however be made between these two surfaces.

The separation or stripping of the cured photoresist from the machinedmechanical structure generally takes place by chemical etching or by aplasma treatment. In this way, the machined metal structure is freed.

The process according to the invention for fabricating a monolayer metalstructure on a bulk metal substrate has many advantages over the knownprocesses for fabricating such structures on a silicon support wafer.

Firstly, it allows any machining operation to be carried out on theupper surface of the structure obtained after electroforming, beforedetachment of the substrate and separation of the cured photoresistmold, especially its leveling by abrasion and optionally polishing so asto obtain a plane surface parallel to the surface of the substrate, andone or more other machining operations such as, for example, beveling,etching or decorative machining. Thus, when fabricating a large numberof structures (or parts) on one and the same substrate, it is possiblefor these structures, positioned and immobilized on said substrate, tobe machined collectively, thus benefiting in the case of this machining,from the positioning and immobilization on extremely precise curedphotoresist molds, carried out during execution of the UV-LIGAtechnique. It is very difficult, if not impossible, to obtain suchprecision in positioning and immobilizing each metal structure after ithas been detached from the substrate.

The process of the invention may be used to fabricate monolayer metalstructures that include an object inserted with extremely precisepositioning thereof.

This is because it is possible, after having carried out steps a) to e),to place a removable fastened object on top of the bulk metal substrateand, when detaching the electroformed metal structure from the bulkmetal substrate by delamination, to free this object from the bulk metalsubstrate. For example, the object may be a jewel, such as a bearingruby of a watch geartrain. The fact of using a bulk metal substrateallows this bearing to be positioned very precisely by means of a pegthat is calibrated to the diameter of the opening for guiding thebearing and is fastened in the substrate, projecting from its surface,so as to allow it to be engaged in the opening for guiding the bearing,a short distance from its surface. During step f), this bearing may beheld captive in the electroformed metal or alloy that surrounds it. Uponreleasing the electroformed structure from the bulk metal substrateduring step i), an electroformed structure that includes an insertedobject (or insert) is thus obtained.

The precise positioning and removable attachment of an object is notpossible on top of a silicon support wafer covered with a sacrificialmetal layer and with a seed layer. The known processes for fabricatingmonolayer metal structures by UV-LIGA use such a wafer and therefore donot allow an object to be inserted into the electroformed metalstructure.

The invention therefore also relates to a novel monolayer machined metalstructure that includes an inserted object, which structure can beobtained by the process defined above.

The process of the invention also allows the manufacture of monolayermetal structures that include a precisely positioned threaded hole.

This is because it is possible, after having carried out steps a) to e),to place a screw 3 in a tapped hole in the bulk metal substrate 1 bymaking this screw 3 project beyond the upper surface of the substrate(cf. FIGS. 5A, 5B) and to unscrew this screw from the bulk metalsubstrate before the metal substrate is detached by delamination. Thescrew consists of an inert material that does not bond to theelectroformed metal, for example Teflon® (PTFE). Below the point whereit is desired to position the threaded hole, a threaded hole is tappedin the bulk metal substrate 1 so as to pass vertically through it and ascrew 3 is introduced into said hole from below (cf. FIGS. 5A, 5B), partof said screw projecting above the substrate. During step f), the metalor alloy will surround that part of the screw projecting from the bulkmetal substrate. Before step i), this screw is unscrewed from the bulkmetal substrate. Depending on the length of this screw, it is possibleto have either a blind tapping (FIG. 5A) or a through-tapping (FIG. 5B).

It is not possible to tap into a silicon support wafer as it is toobrittle and would break. The known processes for fabricating monolayermetal structures by UV-LIGA use such a wafer: they therefore do not astructure having a threaded hole to be fabricated.

The invention therefore also relates to a novel monolayer machined metalstructure that includes a threaded hole, which can be obtained by theprocess defined above.

The invention also relates to a process for fabricating in UV-LIGAtechnology a multilayer machined metal structure having entirelysuperposed layers, which comprises the following steps:

-   -   a) a bulk metal substrate is coated with a photoresist layer;    -   b) if necessary, the photoresist layer is heated in order to        evaporate the solvent;    -   c) the photoresist layer is exposed to UV irradiation of 100 to        2000 mJ/cm² measured at a wavelength of 365 nm, through a mask        corresponding to the desired impression;    -   d) if necessary, in order to complete the photocuring or the        photodecomposition, the layer obtained after step c) is        annealed;    -   e) development takes place by dissolving the uncured or        photodecomposed parts;    -   f) a metal or an alloy is electrodeposited in the open parts of        the photoresist mold;    -   g) leveling is carried out by machining, so as to obtain a plane        upper surface;    -   h) steps a), b), c), d) and e) are repeated and that surface of        the electroformed metal which is not covered with cured        photoresist is activated by an electrochemical treatment;    -   i) steps f) and g) are repeated;    -   j) if necessary, steps h) and i) are repeated;    -   k) if necessary, one or more other machining operations are        carried out on the upper face of the electroformed mechanical        structure; and    -   l) the metal structure and the cured photoresist are detached        from the bulk metal substrate by delamination and the cured        photoresist is separated from the multilayer machined metal        structure having superposed layers.

The expression “multilayer having entirely superposed layers” meansthat, for two adjacent layers, the outline of the upper layer fallsentirely within the vertical elevation of the outline of the lowerlayer.

Steps a), b), c), d), e), f) and g) are the same as those of the processdescribed above for fabricating a monolayer metal structure by UV-LIGA.

Step h) consists in repeating steps a), b), c), d) and e) and inactivating that surface of the electroformed metal which is not coveredwith cured photoresist by an electrochemical treatment.

Steps a), b), c), d) and e) are repeated using, in step a), assubstrate, the plane upper surface obtained after step g) and, in stepc) a new mask corresponding to the desired impression for the newelectroformed metal layer.

That surface of the electroformed metal not covered with curedphotoresist after repeating step e) is for example activated by applyinga reverse current, making the electroformed metal act as anode, usingtechniques well known in the surface treatment art.

Step i) consists in repeating steps f) and g). A metal or an alloy iselectrodeposited in the open part of the new cured photoresist moldobtained after repeating step e), and a machining leveling operation iscarried out, in general by abrasion and polishing, so as to obtain aplane upper surface (with surface irregularities generally not exceedingabout 1 μm). The metal or alloy electrodeposited during step i) may beidentical to or different from the metal or alloy electrodepositedduring step f). In general, it is the same metal or alloy.

Step j) consists, if necessary in order to obtain the desired multilayermetal structure, in repeating steps h) and i). This repetition isunnecessary in the case of fabricating a bilayer metal structure.

Step k) consists, if necessary in order to obtain the desired structure,in carrying out other machining operations on the upper face of theelectroformed mechanical structure, such as for example beveling,etching or decorative machining.

Step l) consists in detaching the machined metal structure from the bulkmetal substrate by delamination, and in separating the photoresist fromthe machined mechanical structure.

The lower face of the metal structure detached from the upper face ofthe bulk metal substrate reproduces the surface finish of said upperface. Thus, it will either be textured (if the upper face of the metalsubstrate is textured, for example by etching or micropeening) or have apolished appearance (if the upper face of the metal substrate hasundergone a polishing operation). In the latter case, by examinationwith the naked eye, the polished appearance of the surface of the lowerface of the structure cannot be distinguished from the polishedappearance obtained where appropriate by polishing on the surface of theupper face. Under an optical microscope, with a magnification of 50times, suitable illumination and a certain orientation, or under ascanning electron microscope or using topographic surface analysissystems, these two surfaces can however be distinguished.

The separation or stripping of the cured photoresist from the machinedmechanical structure generally takes place by chemical etching or plasmatreatment. The machined metal structure is thus freed.

The process for fabricating a multilayer metal structure having entirelysuperposed layers on a bulk metal substrate according to the inventionhas many advantages over the known processes for fabricating suchstructures on a silicon support wafer.

It allows any machining operation to be carried out on the upper surfaceof the metal structure obtained after the final electroforming step,before detachment from the substrate, especially a beveling, etching ordecorative machining operation. Thus, it is possible, when fabricating alarge number of structures (or parts) on any one substrate, for thesestructures positioned and immobilized on the substrate to becollectively machined, benefiting, for this machining, from thepositioning and immobilization on extremely precise cured photoresistmolds, carried out during execution of the UV-LIGA technique. It is verydifficult to obtain such precision in the positioning and immobilizationof each metal structure when this is detached.

The process of the invention includes a leveling step by machining(abrasion and polishing) at the end of each electrodeposition step so asto obtain a plane upper surface. This has the effect of improving thequality of the multilayer metal structure obtained and the regularity ofits thickness, especially certain mechanical properties and/orappearance, since the subsequent electroplating takes place in a moreregular manner and allows better thickness control on a polished surfacethan on a surface having irregularities.

A multilayer machined metal structure having entirely superposed layersthat is novel and has advantageous properties is thus obtained.

The invention therefore also relates to the multilayer machined metalstructure having entirely superposed layers that can be obtained by theprocess defined above.

The process of the invention also makes it possible, thanks to the useof a bulk metal substrate, to obtain novel multilayer metal structureshaving entirely superposed layers which include an inserted object or athreaded hole in the first electroformed layer, by carrying out theprocess in the same way as that described above in the case of themonolayer metal structures. Such multilayer structures that include aninserted object or a threaded hole cannot be obtained by the knownprocesses for manufacturing multilayer metal structures by UV-LIGA thatuse a silicon support wafer.

The invention thus also relates to a multilayer machined metal structurehaving, in its first layer, an inserted object or a threaded hole thatcan be obtained by the process defined above.

Other features and advantages of the invention will become apparent onreading the following detailed description with reference to theappended drawings which illustrate, schematically and by way of example,a few methods of implementing the process of the invention.

In these drawings:

FIG. 1 is a perspective view of a return spring having a monolayerstructure;

FIGS. 2A to 2F are sectional views on the line AB of FIG. 1, showing thevarious steps in the fabrication of the spring of FIG. 1;

FIG. 3A is a perspective view from below and FIG. 3B is a sectional viewon the line AB of FIG. 3A of an anchor for a watch movement escapement;

FIGS. 4A to 4H are sectional views showing schematically the varioussteps in the fabrication of the anchor of FIGS. 3A and 3B;

FIGS. 5A, 5B are partial sectional views of an assembly comprising ascrew 30 in a tapped hole in the bulk metal substrate 5 in order to forma tapping in the electroformed structure; and

FIG. 6 is a partial sectional view showing how a bearing can be insertedinto an electroformed metal layer.

The following examples describe the fabrication of the return springusing the process of the invention, with reference to FIGS. 1 to 4H.

EXAMPLE 1 Fabrication of a Return Spring

FIG. 1 shows a return spring 1, a cylindrical hole 3 and a beveled part4.

FIG. 2A shows the structure obtained after step b) of the process ofclaim 2, which comprises a photoresist layer 6 covering the substrate 5.This structure was obtained using the protocol described below.

A substrate 5 formed from a stainless steel plate 1 mm in thickness and150 mm in diameter was degreased and prepared for the electroforming, bydegreasing with an alkaline solution, by neutralization with an acidsolution, to passivate its surface, and then by rinsing with distilledwater and drying. Next, a negative photoresist first layer, based on theoctofunctional epoxy resin SU-8-2035 (Shell Chemical) with a thicknessof 100 μm was deposited on the substrate 5 by spin coating, followed byheating, to evaporate the solvent, for 5 minutes at 65° C. and then 20minutes at 95° C. Next, a second layer of the same photoresist with athickness of 100 μm was deposited on the first photoresist layer by spincoating, followed by heating, to evaporate the solvent, for 5 minutes at65° C. and then 45 minutes at 95° C.

FIG. 2B corresponds to step c) of the process, with UV illumination ofabout 500 mJ/cm² centered on 365 nm of the photoresist through a maskcorresponding to the desired impression. This figure shows the mask,comprising a UV-transparent support 7 and opaque zones 7 a formed bychromium deposits. The same mask-forming support may have a large numberof zones corresponding to as many structures as can be fabricated in asingle batch, all the zones being obtained with very high resolution ofthe outline by photolithography, a technique well known in themicroelectronics industry.

This UV irradiation R induces photocuring of the resin in the exposedzones 6 b, the unexposed zones 6 a remaining uncured.

FIG. 2C shows the structure obtained after step e) of the process. Thelayer obtained after step c) was annealed, in order to complete thecure, for 1 minute at 65° C. followed by 15 minutes at 95° C., and thenthe unexposed photoresist was dissolved by passing it for minutesthrough three successive PGMEA baths (of increasing purity), rinsing inan isopropyl alcohol bath and drying. FIG. 2C shows the curedphotoresist mold 6 b superposed on the substrate 5.

FIG. 2D shows the structure obtained following step f) of the process,after nickel electrodeposition, then leveling by abrasion and polishingso as to obtain a plane upper surface. This figure shows the cured resinmold 6 b and the electroformed layer 9 that cover the substrate 5.

FIG. 2E shows the structure obtained during step h) of the process,during a beveling operation. This figure shows the substrate 5, themetal structure 9, the cured resin mold 6 b, the beveled hole 3 a andthe milling cutter 8 used for the beveling.

FIG. 2F, which corresponds to the sectional view of FIG. 1, shows thespring obtained following step i) of claim 2, after detachment from themetal substrate by delamination and stripping of the cured photoresistwith N-methylpyrrolidone.

EXAMPLE 2 Fabrication of an Escapement Anchor for a Watch Movement

FIGS. 3A and 3B show an anchor comprising a cylindrical hole 3, a fork 2a and a recess 2 b.

FIG. 4A shows the structure obtained following step b) of the process,which comprises a photoresist layer 6 covering the substrate 5. Thisstructure was obtained using the protocol described below.

A substrate 5 formed from a stainless steel plate 1 mm in thickness and150 mm in diameter was degreased and prepared for the electroforming, bydegreasing with an alkaline solution, by neutralization with an acidsolution, to passivate its surface, and then by rinsing with distilledwater and drying. Next, a negative photoresist first layer, based on theoctofunctional epoxy resin SU-8-2035 (Shell Chemical) with a thicknessof 70 μm was deposited on the substrate 5 by spin coating, followed byheating, to evaporate the solvent, for 3 minutes at 65° C. and then 9minutes at 95° C. Next, a second layer of the same photoresist with athickness of 70 μm was deposited on the first photoresist layer by spincoating, followed by heating, to evaporate the solvent, for 5 minutes at65° C. and then 35 minutes at 95° C.

FIG. 4B corresponds to step c) of the process, with UV illumination ofabout 450 mJ/cm² centered on 365 nm of the photoresist through a maskcorresponding to the desired impression. This figure shows the mask,comprising a UV-transparent support 7 and opaque zones 7 a formed bychromium deposits. This UV irradiation R photocures the resin in theexposed zones 6 b, the unexposed zones 6 a remaining uncured.

FIG. 4C shows the structure obtained after step e) of the process. Thelayer obtained after step c) was annealed, in order to complete thecure, for 1 minute at 65° C. followed by 15 minutes at 95° C., and thenthe unexposed photoresist was dissolved by passing it for minutesthrough three successive PGMEA baths (of increasing purity), rinsing inan isopropyl alcohol bath and drying. This figure shows the curedphotoresist mold 6 b on the substrate 5.

FIG. 4D shows the structure obtained after having carried out steps f)and g) of the process for electroplating nickel in the open parts of thecured photoresist mold and leveling, by abrasion and polishing, so as toobtain a plane upper surface, and having repeated steps a) and b) withtwo successive 50 μm layers of the same photoresist based on epoxy resinSU-8-2035, heating for 3 minutes at 65° C. then 6 minutes at 95° C., inthe case of the first layer, and heating for 5 minutes at 65° C. then 20minutes at 95° C., in the case of the second layer. FIG. 4D shows thesecond photoresist layer 11 covering the cured photoresist 6 b and theelectroformed layer 10, on top of the substrate 5.

FIG. 4E corresponds to the repetition of step c) of the process (duringstep h)), with UV illumination of about 400 mJ/cm² centered on 365 nm ofthe photoresist through a new mask corresponding to the desiredimpression. This figure shows the mask comprising a secondUV-transparent support 12 and opaque zones 12 a formed by chromiumdeposits. This UV irradiation R photocures the resin in the exposedzones 11 b, the unexposed zones 11 a remaining uncured.

FIG. 4F shows the structure obtained following the repetition of step e)of the process (during step h)). The layer obtained following therepetition of step c) was annealed, in order to complete the cure, for 1minute at 65° C., then 15 minutes at 95° C., and then the unexposedphotoresist was dissolved by passing through three successive PGMEAbaths (of increasing purity) for 15 minutes, rinsing in an isopropylalcohol bath and drying. This figure shows the second cured photoresistmold 11 b on top of the first cured photoresist mold 6 b and theelectroformed metal layer 10, on top of the substrate 5.

FIG. 4G shows the structure obtained following the repetition of stepsf) and g) of the process (during step i)). A second electroplating stepwas carried out with the same metal, namely nickel, in an amountslightly greater (10 to 30 μm) than the intended thickness, followed byleveling by abrasion and polishing so as to obtain a plane uppersurface. This figure shows the second cured resin mold 11 b and thesecond electroformed layer 13, on top of the first cured mold 6 b andthe first electroformed layer 10, on top of the substrate 5.

FIG. 4H, which corresponds to the sectional view of FIG. 3B, shows theanchor obtained following step 1) of the process, after the metalsubstrate has been detached by delamination and the cured photoresisthas been removed by a plasma treatment.

FIG. 6 shows the removable positioning of an insert 24, consisting of awatch movement bearing jewel intended to be held captive in the metallayer 22 deposited by electroforming on the metal substrate 21. For thispurpose, a positioning member 23 that terminates in a positioning peg 23a calibrated to the diameter of the hole in the jewel bearing 24, isforced into a positioning hole la in the substrate 21, making the peg 23a project beyond the surface of the substrate 21. The jewel may thus bepositioned precisely and removably on the peg 23 a. The lateral surfaceof the jewel 24 has a groove 24 a so as to allow the metal deposited byelectroforming to form an annular rib 22 a for anchoring the bearing inthe electroformed metal layer 22. The precision thus obtained isparticularly high, since the jewel 24 is centered by its hole and not byits periphery. Furthermore, since the substrate 21 can be reused, it canbe manufactured with extremely close tolerances.

1. A metal structure which is a monolayer metal structure or amultilayer metal structure having entirely superposed layers, whereinthe metal structure has an insert attached thereto, wherein the metalstructure has been obtained by a process for fabricating at least onemonolayer or multilayer metal structure in LIGA technology, comprising:depositing a photoresist layer on a flat metal substrate, creating atleast one photoresist mold by irradiation or electron or ionbombardment, electroplating a metal or alloy in the mold to form anelectroformed metal structure, detaching the electroformed metalstructure and the photoresist layer from the substrate, and separatingthe photoresist from the electroformed metal structure, wherein apositioning member is fastened to the substrate before forming saidelectroformed metal structure, wherein the positioning member positionsan insert on said substrate, wherein said insert is removably associatedwith said positioning member, wherein said positioned insert becomesattached to said electroformed metal structure when said electroformedmetal structure is formed, and wherein the electroformed metal structureis detached from the substrate together with said insert attached to themetal structure and the photoresist layer, so that the obtained metalstructure has the positioned insert attached thereto.
 2. The metalstructure as claimed in claim 1, wherein the fabrication processcomprises the following steps: a) coating a bulk metal substrate with aphotoresist layer; c) exposing the photoresist layer to UV irradiationof 100 to 2000 mJ/cm² measured at a wavelength of 365 nm, through a maskcorresponding to the desired impression; e) carrying out development bydissolving the uncured or photodecomposed parts; f) electrodepositing ametal or an alloy in the open parts of the photoresist mold, g)levelling the electroformed metal structure by machining, so as toobtain a plane upper surface; i) detaching the metal structure and thecured photoresist, by delamination from the bulk metal substrate, thenseparating the cured photoresist from the machined monolayer metalstructure, wherein the positioned insert is fastened to the substrateafter steps a) to e).
 3. The metal structure as claimed in claim 1 whichis a multilayer machined metal structure having entirely superposedlayers in UV-LIGA technology, wherein the fabrication process comprisesthe following steps: a) coating a bulk metal substrate with aphotoresist layer; c) exposing the photoresist layer to UV irradiationof 100 to 2000 mJ/cm² measured at a wavelength of 365 nm, through a maskcorresponding to desired impression; e) carrying out development bydissolving uncured or photodecomposed parts; f) electrodepositing ametal or an alloy is electrodeposited in open parts of the photoresistmold, wherein: g) levelling the electroformed metal structure bymachining, so as to obtain a plane upper surface; h) repeating steps a),c), and e) and activating that surface of the electroformed metal whichis not covered with cured photoresist; i) repeating steps f) and g); l)detaching the metal structure and the cured photoresist from the bulkmetal substrate by delamination from the bulk metal substrate, thenseparating the cured photoresist from the multilayer electroformed metalstructure having superposed layers.
 4. The metal structure as claimed inclaim 1, in which the metal substrate is made of stainless steel.
 5. Themetal structure as claimed in claim 1, in which the surface of thesubstrate has been worked by micropeening, chemical or mechanicaletching, or by a laser so as to obtain the negative of a finishedsurface of the electroformed metal structure.
 6. The metal structure asclaimed in claim 1, in which the metal substrate has an upper facepolished to the degree of polishing desired for the adjacent face ofsaid electroformed metal structure.
 7. The metal structure as claimed inclaim 1, in which, after the metal structure and the cured photoresisthave been detached, etching, surface treatment and mechanical or lasermarking operations have been carried out on the detached metalstructure.
 8. The metal structure as claimed in claim 2, wherein thefabrication process further comprises the following step: b) heating thephotoresist layer in order to evaporate the solvent.
 9. The metalstructure as claimed in claim 2, wherein the fabrication process furthercomprises the following step: d) annealing the layer obtained after stepc) in order to complete the photocuring or the photodecomposition. 10.The metal structure as claimed in claim 3, wherein the fabricationprocess further comprises the following step: b) heating the photoresistlayer in order to evaporate the solvent.
 11. The metal structure asclaimed in claim 3, wherein the fabrication process further comprisesthe following step: d) annealing the layer obtained after step c) inorder to complete the photocuring or the photodecomposition.
 12. Themetal structure as claimed in claim 3, wherein the fabrication processfurther comprises the following steps: j) repeating steps h) and i). 13.The metal structure as claimed in claim 1, wherein the fabricationprocess further comprises the following step: after detaching the metalstructure and the cured photoresist from the bulk metal substrate bydelamination and before separating the cured photoresist from themonolayer structure, carrying out at least one etching,surface-treatment, mechanical or laser marking operation on the detachedmetal structure.
 14. The metal structure as claimed in claim 3, whereinthe fabrication process further comprises the following step: afterdetaching the metal structure and the cured photoresist from the bulkmetal substrate by delamination and before separating the curedphotoresist from the machined multilayer structure, carrying out atleast one etching, surface-treatment, mechanical or laser markingoperation on the detached metal structure.
 15. The metal structure asclaimed in claim 1, wherein the fabrication process further comprisesthe following step: passivating the bulk metal substrate beforedepositing the photoresist layer.
 16. The metal structure as claimed inclaim 15, wherein the metal substrate on which the photoresist layer isdeposited is a bulk metal substrate which has been passivated byalkaline degreasing followed by acid neutralization.
 17. The metalstructure as claimed in claim 1, wherein the bulk metal substrate hasbeen passivated by alkaline degreasing followed by acid neutralization.18. The metal structure as claimed in claim 1, wherein the insert is abearing.
 19. The metal structure as claimed in claim 18, which is anescapement anchor for a clock movement.
 20. The metal structure asclaimed in claim 1, which is an escapement anchor for a clock movement.21. The metal structure as claimed in claim 1, comprising: creating aplurality of photoresist molds by irradiation or electron or ionbombardment, electroplating a metal or alloy in these molds to formelectroformed metal structures, carrying out one or more machiningoperations on the upper face of the electroformed metal structures,detaching the electroformed metal structures and the photoresist layerfrom the substrate, and separating the photoresist from these metalstructures, wherein the bulk metal substrate on which the photoresistlayer is deposited is a passivated bulk metal substrate for separatingthe metal structures and the photoresist from the bulk metal substrateby delamination.
 22. A metal structure which is a monolayer metalstructure or a multilayer metal structure having entirely superposedlayers, wherein the metal structure includes a threaded hole, whereinthe metal structure has been obtained by a process for fabricating atleast one monolayer or multilayer metal structure in LIGA technology,comprising: depositing a photoresist layer on a flat metal substrate,creating at least one photoresist mold by irradiation or electron or ionbombardment, electroplating a metal or alloy in the mold to form anelectroformed metal structure, detaching the electroformed metalstructure and the photoresist layer from the substrate, and separatingthe photoresist from the electroformed metal structure, wherein a screwis placed in at least one tapped hole made through the metal substratebefore the electroformed metal structure is formed, so that a portion ofthe screw extends beyond the surface of the substrate, wherein theelectroformed metal structure is formed directly surrounding the portionof the screw extending beyond the surface of the substrate, and wherein,before detaching the metal structure and the cured photoresist from themetal substrate, the screw is unscrewed, so that the metal structure hasat least one threaded hole formed therein.
 23. The metal structure asclaimed in claim 22, wherein the fabrication process comprises thefollowing steps: a) coating a bulk metal substrate with a photoresistlayer; c) exposing the photoresist layer to UV irradiation of 100 to2000 mJ/cm² measured at a wavelength of 365 nm, through a maskcorresponding to the desired impression; e) carrying out development bydissolving the uncured or photodecomposed parts; f) electrodepositing ametal or an alloy in the open parts of the photoresist mold, g)levelling the electroformed metal structure by machining, so as toobtain a plane upper surface; i) detaching the metal structure and thecured photoresist, by delamination from the bulk metal substrate, thenseparating the cured photoresist from the machined monolayer metalstructure, wherein the screw is placed in the at least one tapped holemade through the metal substrate after steps a) to e).
 24. The metalstructure as claimed in claim 22 which is a multilayer machined metalstructure having entirely superposed layers in UV-LIGA technology,wherein the fabrication process comprises the following steps: a)coating a bulk metal substrate with a photoresist layer; c) exposing thephotoresist layer to UV irradiation of 100 to 2000 mJ/cm² measured at awavelength of 365 nm, through a mask corresponding to desiredimpression; e) carrying out development by dissolving uncured orphotodecomposed parts; f) electrodepositing a metal or an alloy iselectrodeposited in open parts of the photoresist mold, wherein: g)levelling the electroformed metal structure by machining, so as toobtain a plane upper surface; h) repeating steps a), c), and e) andactivating that surface of the electroformed metal which is not coveredwith cured photoresist; i) repeating steps f) and g); l) detaching themetal structure and the cured photoresist from the bulk metal substrateby delamination from the bulk metal substrate, then separating the curedphotoresist from the multilayer electroformed metal structure havingsuperposed layers.
 25. The metal structure as claimed in claim 22, whichis an escapement anchor for a clock movement.
 26. The metal structure asclaimed in claim 22, comprising: creating a plurality of photoresistmolds by irradiation or electron or ion bombardment, electroplating ametal or alloy in these molds to form electroformed metal structures,carrying out one or more machining operations on the upper face of theelectroformed metal structures, detaching the electroformed metalstructures and the photoresist layer from the substrate, and separatingthe photoresist from these metal structures, wherein the bulk metalsubstrate on which the photoresist layer is deposited is a passivatedbulk metal substrate for separating the metal structures and thephotoresist from the bulk metal substrate by delamination.